1. Field of the Invention
The present invention generally relates to a waste water treating apparatus. More specifically, the present invention relates to a waste water treating apparatus for purifying domestic waste water or industrial waste water containing organic matter and ammonium nitrogen by treatment with microorganisms.
2. Description of the Prior Art
A general approach for treating waste water comprises a primary treating step for physically removing suspended matter of a given size from the waste water and a secondary treating step for removing organic matter by a biological process. The present invention is directed to an improvement in the secondary processing step.
Waste water being treated in the above described secondary treating step comprises organic matter including protein, fat, carbohydrate, amino acid or the like and nitrogen or the like. In order to perform the above described secondary treating step of nitrogen containing waste water, a conventional approach first removes the BOD material biochemical oxygen demand through decomposition thereof, which is followed by an oxidation of ammonium nitrogen for conversion into ions of nitrite or nitrate, which is further followed by denitrification by adding an organic carbon source such as methanol or the like under a reduction atmosphere having a low concentration of oxygen.
FIG. 1 is a flow diagram showing a conventional example of a most fundamental waste water treating apparatus utilizing a biological denitrification method, which constitutes the background of the invention. Referring to FIG. 1, waste water containing organic matter and nitrogen and having suspended matters removed is supplied into a nitrification tank 1. The organic nitrogen and ammonium nitrogen (NH.sub.4 -N) contained in the waste water is oxidized in the nitrification tank 1 by means of a nitrifying microorganism and oxygen to produce nitrite nitrogen (NO.sub.2 -N) or nitrogen (NO.sub.3 -N). The waste water thus subjected to nitrification, i.e. a mixed liquor, is fed from the nitrification tank 1 to a denitrification tank 2. In the denitrification tank 2 nitrogen (N.sub.2) is separated by means of respiration of a denitrifying microorganism. The nitrogen thus separated is discharged into the atmosphere in the form of a gas. A treated water thus denitrified in the denitrification tank 2 is further introduced into a precipitation tank 3. The treated water is separated in the precipitation tank 3 into an activated sludge and water. The treated water from the precipitation tank 3 is subjected to primary type process or it is discharged out of the system after being subjected to a chlorine sterilization. A portion of the activated sludge obtained in the precipitation tank 3 is returned so that the same is introduced into the nitrification tank 1 together with the waste water.
FIG. 2 is a flow diagram showing another conventional example which also constitutes the background of the invention. The apparatus of FIG. 2 differs from that of FIG. 1 in that in FIG. 2 the apparatus additionally comprises a BOD oxidation tank 4 interposed between the denitrification tank 2 and the precipitation tank 3. The apparatus of FIG. 2 is disclosed in Japanese Patent Publication No. 31226/1974 published for opposition Aug. 20, 1974, for example. The BOD oxidation tank 4 functions to oxidize the remaining BOD material with a BOD oxidizing microorganism, thereby to stabilize the treated water quality of water discharged from the precipitation tank 3.
Any of the above described two conventional approaches requires separate tanks for both the nitrification and denitrification processes. Accordingly, these conventional approaches necessitate large scale facilities including separate tanks of large capacity, which accordingly require a site of a large area for installation. In addition, in the nitrification tank 1 ammonium nitrogen is oxidized to produce nitrate or nitrite, with the result that the pH value is considerably decreased. When exposed to a lower pH value a nitrifying microorganism is much less active and it becomes necessary to add alkalis such as caustic soda for adjusting the pH value. Furthermore, since a heterotrophic microorganism is generally utilized as a denitrifying microorganism in the denitrification process, it is necessary to add a large amount of an organic carbon source such as methanol. Accordingly, the cost for running such facilities has become expensive.
Japanese Patent Publication No. 38357/1975 laid open for public inspection on Apr. 9, 1975 proposes a modification of the approach shown in FIG. 1, in which a plurality of stages for the nitrification tanks and for the denitrification tanks are provided, so that the dose of a pH value adjusting agent such as caustic soda is reduced as much as possible. The approach disclosed in Japanese Patent Publication No. 38357/1975 aims at reducing as much as possible the use of an alkali for neutralizing by repeating the nitrification and denitrification, thereby to reduce the running costs and the concentration of the salt groups in the treated water. However, the above described modification of the approach shown in FIG. 1 still requires large scale facilities. In addition, the above described modification of the FIG. 1 approach still requires a dosage of organic carbon such as methanol in the denitrification process just as in FIGS. 1 and 2, whereby the operating costs cannot be sufficiently reduced.
FIG. 3 is a flow diagram showing a further conventional example also forming part of the background of the invention. The example of FIG. 3 is disclosed in Japanese Patent Publication No. 113047/1975 laid open for public inspection on Sept. 4, 1975. The conventional example of FIG. 3 has eliminated the shortcoming involved in the above described conventional examples that a large amount of organic carbon source such as methanol need be dosed in the denitrification process, whereby the amount of an external hydrogen donor has been reduced as much as possible. Briefly described, the example of FIG. 3 is characterized by a first denitrification tank 2a and a second denitrification tank 2b respectively located upstream and downstream of the nitrification tank 1. The BOD substance contained in the waste water is oxidized to be decomposed in the first denitrification tank 2a by means of nitrite and nitrate respiration. Accordingly, the required amount of oxygen in the nitrification tank 1 is correspondingly decreased. A portion of the mixed liquor flowing out from the nitrification tank 1 is returned to the first denitrification tank 2a. The remaining nitrite nitrogen and nitrate nitrogen are fully reduced to be decomposed in the second denitrification tank 2b. The activated sludge obtained from the precipitation tank 3 is returned to the second denitrification tank 2b. The conventional example of FIG. 3 makes it possible to dispense with or to reduce the dose of the hydrogen donor or of the organic carbon to an extremely small amount for the denitrification process, which accordingly decreases the operating costs. However, even the conventional example of FIG. 3 still requires at least one nitrification tank and two denitrification tanks, with the resultant problem that facilities of a large scale are involved. In addition, even the conventional example of FIG. 3 cannot fully remove the BOD component in the first denitrification tank 2a, with the result that a large amount of the remaining BOD component enters into the nitrification tank 1. Therefore, the BOD oxidizing microorganism increases and accordingly the sludge amount increases, which decreases the average residence time period of the sludge and also decreases the rate of inclusion of the nitrifying microorganism in the sludge, whereby it becomes difficult to maintain the nitrifying microorganism in the nitrification tank, particularly at a lower temperature. In order to maintain the nitrifying microorganism under such condition, it becomes necessary to increase the capacity of the nitrification tank 1.
FIG. 4 is a flow diagram showing still another conventional example providing further background of the invention. The conventional example of FIG. 4 is disclosed in Japanese Patent Publication No. 79573/1977 laid open for public inspection of July 4, 1977. FIG. 4 is characterized by an oxidation tank 5 between the first denitrification tank 2a and the nitrification tank 1. The remaining BOD component is removed from the oxidation tank 5, thereby to decrease the BOD component entering into the nitrification tank 1. Although the conventional example of FIG. 4 can enhance the denitrification efficiency, the same requires a further separate tank and accordingly involves a problem that the facilities become unavoidably large.
FIG. 5 is a flow diagram of yet another conventional example providing further background of the invention. The conventional example of FIG. 5 is disclosed in Japanese Patent Publication No. 42850/1979 laid open for public inspection on Apr. 5, 1979. The conventional example of FIG. 5 is owned by the assignee of the present invention and has been marketed under the trademark "Kubota Nitrocycle System". The conventional system of FIG. 5 comprises an aerobic digestion tank 6 located upstream of an aeration and nitrification tank 1 and a reaeration tank 1a located between the denitrification tank 2 and the precipitation tank 3. Not only a portion of the mixed liquor flowing out from the aeration tank 1 but also the activated sludge from the precipitation tank 3 are returned to the aeorobic digestion tank 6. The conventional system of FIG. 5 functions such that the BOD oxidation and nitrification take place simultaneously through processing of the activated sludge in the aerobic digestion tank 6 and in the aeration an nitrification tank 1, for removing a major portion of the total nitrogen (T-N) contained in the waste water and to denitrify the nitrite nitrogen and the nitrate nitrogen in the denitrification tank 2. Accordingly, the conventional system of FIG. 5 makes it possible to drastically reduce the dosage of an alkali (caustic soda) for the nitrification process and the dosage of the organic carbon source (methanol) in the denitrification process. However, the conventional system of FIG. 5 also requires a large supply amount of oxygen containing gas (air) in the aeration tank 1. On the other hand, the efficiency of supplying oxygen into an aeration tank having a depth of 4 to 5 m which is commonly utilized is 5 to 10% at the most and therefore there is a limit to the concentration of the activated sludge that can be maintained in the tank. Recently a circulation type aeration tank having a long path in the vertical direction has been developed to enhance the efficiency of supplying oxygen, in which the oxidizing decomposition and nitrification of the BOD have been considerably improved. However, it is still necessary to carry out the denitrification in a separate reaction tank and accordingly there is room for a fundamental improvement in the implementation of the facilities of small size, and to increase the processing efficiency, as well as reduce the operating costs and the surface area for installing the plant.